This application claims priority of European Patent Application No. 98308310.6, which was filed on Oct. 13, 1998.
The present invention generally relates to the field of optical transmission and particularly to a method and an apparatus for controlling the optical power of an optical transmission signal in a wavelength division multiplex optical transmission system.
Wavelength division multiplexing (WDM) is being introduced as a means of increasing the capacity of optical fibre transmission systems. In a WDM system each individual fibre carries a number of optical signals having different wavelengths. When these optical signals are transmitted over long distances, periodic regeneration of the optical signals is necessary. Currently, this regeneration is effected either by demultiplexing the different wavelengths and then converting the optical signals to corresponding electrical signals which are regenerated and then reconverted to optical signals or by using optical amplifiers, e.g. Erbium Doped Fibre Amplifiers (EDFA). Optical amplifiers have the advantage of both relatively low cost and the ability to amplify all used wavelengths without the need for demultiplexing and opto-electronic regeneration. WDM systems currently under development will have eighty or more channels, i.e. modulated optical signals with different wavelengths (known as Dense Wavelength Division Multiplexing, DWDM).
However, if EDFAs are used for regeneration of the optical transmission signal, a problem arises if one or more channels of the optical transmission signal fail or are added to or dropped from the optical transmission signal as EDFAs are sensitive to variations of the input power. In these cases cross saturation in EDFAs will induce power transients in the surviving channels. The surviving channels will suffer error bursts if, e.g., their powers exceed thresholds for optical nonlinearities or become too low to preserve adequate eye opening.
From xe2x80x9cFAST LINK CONTROL PROTECTION FOR SURVIVING CHANNELS IN MULTIWAVELENGTH OPTICAL NETWORKSxe2x80x9d, by J. L. Zyskind et al, 22nd European Conference on Optical Communicationxe2x80x94ECOC""96, Oslo, pages 49-52, it is known to add one additional channel to the optical transmission signal, for controlling the power of the optical transmission signal. The optical power of the control channel is controlled to keep the total optical power of the optical transmission signal constant, e.g., if a channel of the optical transmission signal fails, the optical power of the control channel is increased to keep the total optical power of the optical transmission signal constant. To change the optical power of the control channel usually the injection current of a laser which generates the control channel is changed as the laser is operated in the continuous wave mode.
As the control channel has to compensate for the power variations of several dozens of channels, the optical power of the control channel can be several times higher than the optical power of a data channel. A non linear effect known as Stimulated Brillouin Scattering (SBS) causes a problem for a not modulated high power optical signal, as used for the control channel, because after the optical power exceeds a threshold, the optical power coupled into the fibre will be reflected within the fibre. The level of that so called Brillouin threshold depends on the material of the fibre used, on the length of the fibre and on the power density spectrum of the optical signal. For an optical signal having a wavelength of 1550 nm and a power density spectrum less than 2 MHz, the Brillouin threshold PT for a fibre having a length of 30 km is 2-10 mW. As the power of the control channel can be substantially higher, the optical signal forming the control channel is more or less completely reflected.
Accordingly, it is an object of the present invention to provide a method and an apparatus for controlling the optical power of an optical transmission signal in wavelength division multiplex optical transmission. It is one aim of the inventive method and the apparatus under consideration to avoid the drawbacks known from the state of the art.
According to a first object of the invention there is provided a method for controlling the optical power of an optical transmission signal in a wavelength division multiplex system, comprising adding an optical control signal having a variable optical power to compensate for variations in the optical power of the optical transmission signal, and spreading an associated power density spectrum of the optical control signal.
According to a second object of the invention there is provided an apparatus for controlling the optical power of an optical transmission signal in a wavelength division multiplex system, having a light source means for generating an optical control signal having a variable optical power, a control means for detecting variations in the optical power of the optical transmission signal and controlling in response the optical power of the light source means, an adding means for adding the optical control signal to the optical transmission signal, and means for spreading an associated power density spectrum of the optical control signal.
An advantage of the present invention is that it allows the optical power of an optical transmission signal to be kept at a constant level with the help of a control channel having an optical signal which has an expanded power density spectrum, thus avoiding negative effects of SBS.
The present invention will become more fully understood from the detailed description given hereinafter and further scope of applicability of the present invention will become apparent. However, it should be understood that the detailed description is given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art.